Selective catalytic reduction (SCR) provides a method for removing nitrogen oxides (NOx) emissions from fossil fuel powered systems for engines, factories, and power plants. During SCR, a catalyst facilitates a reaction between exhaust-gas ammonia and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas.
The ammonia that is used for the SCR system may be produced during the operation of the NOx-producing system or may be stored for injection when needed. Because of the high reactivity of ammonia, storage of ammonia can be hazardous. Further, on-board production of ammonia can be costly and may require specialized equipment.
One method of on-board ammonia production for an engine is disclosed in U.S. Pat. No. 6,047,542, issued to Kinugasa on Apr. 11, 2000 (hereinafter the '542 patent). The method includes the use of multiple cylinder groups for purifying exhaust gas. In the method of the '542 patent, the exhaust gas of one cylinder group may be made rich by controlling the amount of fuel injected into the cylinder group. The rich exhaust gas of this cylinder group may then be passed over an ammonia-synthesizing catalyst to convert a portion of the NOx in the exhaust gas into ammonia. The exhaust gas and ammonia of the first cylinder group are then combined with the exhaust gas of a second cylinder group and passed through an SCR catalyst where the ammonia reacts with NOx to produce nitrogen gas and water.
While the method of the '542 patent may reduce NOx from an exhaust stream through use of on-board ammonia production, the method of the '542 patent has several drawbacks. For example, an engine may function less efficiently and with lower power output when rich combustion occurs in one cylinder group. Furthermore, using the method of the '542 patent, it may be more difficult to provide adequate and controlled air intake to both cylinder groups, and the two cylinder groups, operating as described in the '542 patent, may cause significant engine vibration.
The present disclosure is directed at overcoming one or more of the problems or disadvantages in the prior art.